It is very useful to measure the hardness of a subject. For example, in the case that the subject is human body, it is possible to know the hardness of muscle and internal organs and to determine a degree of progress of arterial sclerosis, and to develop a plan of treatment by measuring a degree of hardness at a predefined position of the body.
For example, Japanese Unexamined Patent Application No. Hei 10-211172 (Patent Document 1) describes that an acceleration sensor and a pressure sensor are pushed against human body, and measuring hardness of the subject (elastic coefficient) based on pressure information and acceleration information which is integrated two times.
As a commonly-used hardness measuring apparatus, there is a durometer. By using the durometer, the hardness of the subject is measured based on the depth of a dent made by a needle which is pushed against the body.